HwBlob: s/malloc/calloc/

Since this blob is passed between processes.

We could potentially only memset portions of the blob as it is
written to. However, the JHwBlob API itself doesn't have to have
writes in order (even though known usages of it do write in order).
Because of this, keeping track of which bytes to pad would be too
expensive.

Bug: 130161842
Test: boot, hidl_test_java
Change-Id: I48f4d7cb20c4bfe747dd323ae3744d323ad097c9